A relay circuit for connecting and disconnecting a quick charger to and from a battery connecting junction circuit during charging has been used in a charging circuit of an electric vehicle. A mechanical relay contact (hereinafter, abbreviated to a relay) is used in the relay circuit, and the relay is turned on and off and is welded when a high voltage is applied and a large amount of current flows. A relay welding detection circuit for detecting the welding of the relay has been known (for example, see Patent Literature 1).
In the related art, when the input-side impedance of the relay welding detection circuit is not a predetermined value (for example, 1 MΩ or less), for example, the problem that a current flows from a quick charger to the welding detection circuit arises. Therefore, it is necessary to increase the input-side impedance of the relay welding detection circuit. In addition, when a detection voltage is insulated and detected, it is necessary to supply driving power to the secondary side.